Copper is widely used in the fabrication of magnetic recording heads because of its high electrical conductivity, and is typically electroplated to produce coil turns, electric leads, and interconnects (studs) between different metal layers in a device. In order to plate copper on a substrate, a thin Cu film with a thickness from 500 to 1000 Angstroms is first sputtered on a wafer surface to serve as a plating seed. Next, a photoresist layer is coated on the Cu seed layer and is patternwise exposed and developed by a conventional lithography method to form a photoresist mask. During the copper plating step, copper is deposited on regions of the substrate that are not protected by the photoresist mask. Subsequently, the photoresist mask is removed by an organic solvent such as NMP. Then, ion milling is used to remove the plating seed in regions previously covered by the photoresist mask to prevent shorting in the resulting devices.
One factor that may affect the quality of a plated copper film is the plating seed layer. In particular, copper is known to be easily oxidized in air to form copper oxide on the surface of copper films. The thickness of copper oxide can vary depending on the dwell time between seed layer deposition and plating, and the nature of up stream process conditions such as ashing for photoresist scum removal prior to copper seed layer activation. Copper oxide may cause poor adhesion of a photoresist mask layer when copper oxide reacts with aqueous base developer during patterning of the photoresist layer. Thus, portions of the protective photoresist mask may lift off or peel to cause defects. In addition, copper oxide can affect surface wettability and thereby degrade plating uniformity. Copper oxide at a copper seed/copper plated film interface can affect coil resistance and hence lower the performance of magnetic recording heads. Therefore, it is very important to remove copper oxide from seed layers before performing copper plating.
Several methods are disclosed in the prior art with regard to copper oxide removal or the so-called activation of the seed layer prior to plating. A commonly used method as disclosed in U.S. Pat. No. 5,482,174 is the application of diluted sulfuric acid or hydrochloric acid, or a combination of one of the aforementioned strong acids with a weak acid. Although this treatment is effective in removing surface copper oxide, a problem is that the copper beneath the oxide surface layer is also etched away by the strong acid. As a result, thinned portions of the copper seed layer may affect plating uniformity. Furthermore, aggressive etching by the strong acid can undercut the photoresist mask and cause lifting of the layer. In extreme cases, strong acids may etch completely through the copper seed layer and attack underlying device features if they are exposed without cathodic protection.
Referring to FIG. 1, an example of an abnormal coil resistance pattern caused by acid activation is depicted. Coil resistance is shown as a function of location (x, y coordinate) on the wafer surface (200 mm diameter). In this example, coil resistance is in the range of 2.8 to 3 ohms in lighter regions 60 around the outer portion of the wafer but is 20% to 30% lower in darker region 61 (left center portion) and in region 62 in the lower right center of the wafer. To overcome this issue, strong acid may be replaced by water vapor which improves the wettability of copper plating seed and eliminates the fish eye issue associated with plating feature devices. However, water vapor is not effective in removing copper oxide and an improved method is still needed that avoids over etching the copper seed layer, improves plating, and reduces the volume of chemical consumption involved with a conventional seed layer activation process which requires as much as 200 ml or more of acid/water mixture per wafer.
Dry etch methods have been practiced in the art as described in U.S. Pat. No. 6,033,584 where a hydrogen plasma treatment is used to remove copper oxide. In U.S. Pat. No. 6,734,102, an in-situ plasma reducing process is disclosed to remove copper oxides and comprises a compound of nitrogen and hydrogen such as ammonia. Unfortunately, plasma tools are quite expensive and typically generate residues that require additional cleaning steps.